If it will still hold and support a load of charcoal I would just hit it with a wire brush. After all it's just a charcoal grate not a food grate.
I don't remember what size charcoal grate the 18.5 WSM uses and not sure if HD would even have it. If you need a new one you may have to get it from Weber. Another option would be to make a charcoal grate out of a piece of expanded metal.